1. Field of the Invention
The present invention is directed to a process for the preparation of polyurethane coatings from two-component coating compositions which have a long potlife, but cure rapidly to form light stable polyurethane coatings.
2. Description of the Prior Art
It is difficult to provide coating compositions which have a long potlife and also cure rapidly to form the final coating. Generally, catalysts are added to the coating composition in order to accelerate the cure rate; however, the catalysts also shorten the potlife of the reaction mixture. One method which has been developed to overcome this difficulty is to cure the coating compositions in an amine vapor chamber. Because the amine catalyst is not present when the reaction components are initially mixed, the mixture has a long potlife. As the mixture passes through the chamber, the gaseous amine vapor catalyzes the reaction resulting in rapid cure. The preparation of coatings using the amine vapor chamber is disclosed in U.S. Pat. Nos. 4,396,647, 4,374,181 and 4,365,039.
One of the difficulties with this preparation procedure is that tertiary amines are only suitable for accelerating reactions between hydroxyl groups and aromatic isocyanate groups. They have relatively little affect on the reaction between hydroxyl groups and aliphatic, cycloaliphatic and araliphatic isocyanate groups. This latter reaction is generally catalyzed by tin compounds; however, since tin catalysts cannot be vaporized in the same manner as amine catalysts, the advantages of the amine vapor chamber cannot be obtained when using tin catalysts.
Accordingly, an object of the present invention is to provide an improved process for preparing polyurethane coatings from two-component compositions based on hydroxyl group-containing compounds and aliphatic, cycloaliphatic or araliphatic polyisocyanates which have a long potlife and yet cure rapidly. An additional object of the present invention is to provide a process for obtaining the benefits of the amine vapor chamber when using aliphatic, cycloaliphatic or araliphatic polyisocyanates instead of aromatic polyisocyanates. These objects may be achieved in accordance with the present invention as described hereinafter.